As is known, a radial tire comprises a tread, two inextensible beads, two sidewalls connecting the beads to the tread, and a belt placed circumferentially between the carcass reinforcement and the tread. This belt is made up of various rubber plies (or “layers”) which may or may not be reinforced by reinforcing elements (“reinforcing threads”) such as cabled threads or monofilaments, of the metal or textile type.
A tire belt generally consists of at least two superposed belt plies, often called “working” plies or “crossed” plies, the generally metallic reinforcing cords of which are placed so as to be practically parallel to one another within a ply, but at an angle from one ply to another, that is to say they are inclined, whether symmetrically or not, to the median circumferential plane by an angle which is generally between 10° and 45° depending on the type of tire in question. The crossed plies may be accompanied by various other auxiliary rubber plies or layers, which vary in width depending on the case and may or may not comprise reinforcing threads. As an example, mention may be made of simple rubber cushions, so-called “protective” plies responsible for protecting the rest of the belt from outer attack or perforations, or else so-called “hoop” plies having reinforcing threads oriented substantially along the circumferential direction (so-called “zero-degree” plies), irrespective of whether they are radially outer or inner to the crossed plies.
As is known, such a tire belt must meet various often contradictory requirements, in particular:                it must be as rigid as possible at low deformation, as it contributes substantially to stiffening the tire crown;        it must have as low a hysteresis as possible, in order, on the one hand, to minimize tire heating of the inner region of the crown during travel and, on the other hand, to reduce the rolling resistance of the tire, synonymous with fuel economy; and        finally, it must have a high endurance, in particular with respect to the phenomenon of separation, i.e. cracking of the ends of the crossed plies in the shoulder region of the tire, known as “cleavage”, which in particular requires metal cords that reinforce the belt plies to have a high compressive fatigue strength, while being in a relatively corrosive atmosphere.        
The third requirement is particularly demanding in the case of tires for industrial vehicles, such as heavy vehicles, which are designed to be retreaded one or more times when their treads reach a critical state of wear after prolonged running.
To reinforce the above belts, it is general practice to use what are called “layered” steel cords formed from a central core and one or more layers of concentric wires placed around this core. The layered cords most often used are essentially cords of M+N or M+N+P construction, formed from a core of M wires surrounded by at least one layer of N wires, said layer itself being optionally surrounded by an outer layer of P wires, the M, N and even, P wires generally having the same diameter for simplification and cost reasons.
The availability of carbon steels of ever greater strength and endurance means that tire manufacturers nowadays are tending, as far as possible, to use cords having only two layers, in particular so as to simplify the manufacture of these cords, to reduce the thickness of the composite reinforcing plies, and thus reduce tire hysteresis, and ultimately to reduce the cost of the tires themselves and the energy consumption of vehicles fitted with such tires.
For all the above reasons, the two-layer cords most often used at the present time in tire belts are essentially cords of 3+N construction formed from a core or inner layer of 3 wires and an outer layer of N wires (for example, 8 or 9 wires). The outer layer is relatively unsaturated thanks to the large diameter of the inner layer provided by the presence of the 3 core wires, the more so when the diameter of the core wires is chosen to be greater than that of the wires of the outer layer.
As is known, this type of construction promotes the penetration of the cord from the outside by the calendering rubber of the tire or other rubber article during the curing thereof, and consequently makes it possible to improve the fatigue and corrosion-fatigue endurance of the cords, particularly as regards the abovementioned cleavage problem.
Moreover, it is known that good penetration of the cord by rubber makes it possible, thanks to a lesser volume of trapped air in the cord, to reduce the tire curing time (“the press time”).
However, cords of 3+N construction have the drawback that they cannot be penetrated right to the core because of the presence of a channel or capillary at the centre of the three core wires, which channel or capillary remains empty after impregnation by the rubber and is therefore propitious, through a kind of “wicking effect”, to the propagation of corrosive media such as water. This drawback of cords with a 3+N construction is well known, being discussed for example in the patent applications WO 01/00922, WO 01/49926, WO 2005/071157 and WO 2006/013077.
To solve the above problem, it has been proposed to open the inner layer Ci, by moving its wires further apart, by means of a single centre wire, and to eliminate one wire from the outer layer. The cord thus obtained, of 1+3+(N−1) construction, can be penetrated from the outside right to its centre. Compared with the wires of the inner layer, the centre wire must be neither too fine, as otherwise the intended desaturation effect would not occur, nor too coarse, as otherwise the wire would not remain at the centre of the cord. It is typical to use, for example, a centre wire of 0.12 mm diameter and Ci and Ce layer wires of 0.35 mm diameter (see for example RD (Research Disclosure) August 1990, No. 316107, “Steel cord construction”).
This solution is firstly expensive, as it requires adding a wire that moreover does not add to the strength of the cord. It also encounters a manufacturing problem: a high tension in the centre wire is necessary in order to keep the wire at the centre of the cord during cabling, which tension may in certain cases approach the tensile strength of the wire. Finally, removing an outer wire has the consequence of further reducing the strength of the cord per unit cross section.
Again to solve this core penetrability problem of 3+N cords, patent application US 2002/160213 proposes to produce cords of the in-situ-rubberized type. The process proposed here consists in individually sheathing (i.e. sheathing in isolation, “wire to wire”) with uncured rubber, upstream of the assembling point of the three wires (or twisting point), just one or preferably each of the three wires in order to obtain a rubber-sheathed inner layer, before the N wires of the outer layer are subsequently put into place by cabling around the thus sheathed inner layer.
The process proposed above poses many problems. Firstly, sheathing just one wire in three (as illustrated for example in FIGS. 11 and 12 of that document) does not ensure that the final cord is filled sufficiently with the rubber compound, and therefore fails to obtain a satisfactory corrosion resistance. Secondly, although wire-to-wire sheathing of each of the three wires (as illustrated for example in FIGS. 2 and 5 of that document) it does actually fill the cord, it results in the use of an excessively large amount of rubber compound. The oozing of rubber compound from the periphery of the final cord then becomes unacceptable under industrial cabling and rubber coating conditions.
Because of the very high tack of uncured rubber, the cord thus rubberized becomes unusable because of it sticking undesirably to the manufacturing tools or between the turns of the cord when the latter is being wound up onto a receiving spool, without mentioning the final impossibility of correctly calendering the cord. It will be recalled here that calendering consists in converting the cord, by incorporation between two uncured rubber layers, into a rubber-coated metal fabric serving as semifinished product for any subsequent manufacture, for example for building a tire.
Another problem posed by individually sheathing each of the three wires is the large amount of space required by having to use three extrusion heads. Because of such a space requirement, the manufacture of cords comprising cylindrical layers (i.e. those with pitches p1 and p2 that differ from one layer to another, or having pitches p1 and p2 that are the same but with twisting directions that differ from one layer to another) must necessarily be carried out in two discontinuous operations: (i) in a first step, individual sheathing of the wires followed by cabling and winding of the inner layer; and (ii) in a second step, cabling of the outer layer around the inner layer. Again because of the high tack of uncured rubber, the winding and intermediate storage of the inner layer require the use of inserts and wide winding pitches when winding onto an intermediate spool, in order to avoid undesirable bonding between the wound layers or between the turns of a given layer.
All the above constraints are punitive from the industrial standpoint and go counter to achieving high manufacturing rates.